| ??? 08/31/05 13:36 Read: times |
#100348 - Problems with LPC922 at start-up Responding to: ???'s previous message |
Hello,
I've just found out about this thread, as I am experiencing problems with LPC922 reset too. I'm developping a board using 2 LPC922, each powered by a "proper" power supply (LM1117), and each monitored by a supervisor. Both the external reset pin (RPE) and Brownout detect (BOE) are enabled. However, the UC don't always start up as expected, and sometimes even seem to be completely "lost". Sometimes they even seem not to start code execution at all. After reading through this thread, I've checked quite a few things: the power supply does drop below 0.2V every time before power-up, the external quartz (10Mhz) starts every time (although with one particular µC, it was taking 10s to start, sometimes... ). What's off-putting is that this behavior seem to be quite random. The only noticeable "defect" in the start-up sequence was a small glitch (<0.8V, <2ms) on the reset line (held low by the supervisor) while the power supply is transitioning from 0V to 3.3V. Also, some uC seem to have more trouble to start-up properly that others. After many experiments, I've come to the hypothesis that this non-deterministic behavior was caused by the reset input (hence by the external supervisor). As a matter of fact, as far as I've tried(100s of start-ups), the board starts up properly every time without these supervisors... what we don't understand is why using the reset input would cause so much trouble? Any suggestion as to what to try is welcome, as removing the supervisor isn't such a great option in our case... Laurent PS : BTW, any idea what that statement (LPC922 datasheet) exactly means: During a power-up sequence, the RPE selection is overridden and this pin will always function as a reset input. An external circuit connected to this pin should not hold this pin LOW during a power-on sequence as this will keep the device in reset. Does that prohibit the use of external supervisor? |
